1. Field of the Invention
This invention concerns a procedure for draining off shallow gas from the seabed and an arrangement for execution of the procedure.
2. Prior Art
In areas of the North Sea and Gulf of Mexico there are strong indications that shallow gas in the upper sedimentary layers percolates upward to the seabed. In relation to construction and installation works on the seabed, pipelines, platforms etc., such areas can be called "active". If precautionary measures are not performed, such areas ought to be avoided with respect to construction and installation works.
In active areas where the upper sedimentary layer consists of soft, fine grained and clayey sediment, the natural gas migration is focused, and may burst through the upper layer in a random pattern. The focusing effect is caused by the semi-impermeability of the upper layer. In soft sediment, shallow depressions termed "pockmarks" are observed, and it is hypothesized that these are formed due to the migration process described above.
A seabed where "pockmarks" are found and where seismic surveys indicate that shallow gas exists in the upper sedimentary layers, may be called "active and instable". It is proposed that precautionary measures be executed in such areas prior to construction and installation works on the seabed. The objective of these precautions shall be to ensure that the area in question becomes inactive and stable, so that there exists no danger that shallow gas will percolate upward beneath the construction.